The metabolism of vitamin D has been greatly elucidated in recent years, with the recognition that it must undergo hydroxylation in one or more locations on the parent molecule to produce active metabolites. The initial hydroxylation occurs in the liver, producing 25-hydroxyvitamin D (25(OH)D), which is the major active metabolite found in plasma. Subsequent hydroxylation in the kidney produces 1,25-dihydroxyvitamin D, which is the most active metabolite yet found to influence intestinal calcium absorption. Although previous studies have successfully clarified much about these steps in metabolism, little is known about the quantitative aspects of this sequence; in particular, there is no information on pool sizes, rate constants, or transport rates. This proposal describes a method which we have developed to quantify in man these parameters of 25(OH)D metabolism by compartmental analysis. Preliminary data indicate that there are probably two 25(OH)D pools, one of which appears to be equivalent to plasma 25(OH)D, and the second of which may represent the biliary and intestinal phase of an active enterohepatic circulation. It is proposed to examine these parameters in various disease states which are associated with abnormalities of vitamin D metabolism, as well as to examine the quantitative effects of perturbations of calcium, phosphate, magnesium, parathyroid hormone, and calcitonin. In subsequent phases, kinetic parameters of the metabolites of 25(OH)D will be examined in a similar manner. It is anticipated that such data will permit a rational approach to therapeutic uses of vitamin D and its metabolites.